1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device.
2. Description of the Related Art
With an advance in scale-down of semiconductor devices, it is difficult to securely fill an isolation trench with an insulating film. The aspect ratio (trench depth/trench width) of the isolation trench is high in a NAND type flash memory in particular. For this reason, it is very difficult to fill the isolation trench using only silicon oxide film (referred to as CVD silicon oxide film, hereinafter) formed by CVD (chemical vapor deposition).
In order to solve the foregoing problem, a method of using perhydrosilazane polymer (referred to as polysilazane) has been proposed (e.g., see JPN. PAT. APPLN. KOKAI Publication No. 2003-258082). Specifically, a polysilazane solution is coated, and thereafter, heat treatments such as a bake treatment (baking), cure treatment (curing) and densify treatment (densifying) are carried out. In this way, a silicon oxide film (SiO2 film) is obtained. The silicon oxide film formed using polysilazane is hereinafter referred to as a polysilazane silicon oxide film, for convenience. For example, a stacked film of CVD silicon oxide film and polysilazane silicon oxide film is used as an isolation insulating film. By doing so, an isolation trench having a high aspect ratio can be filled with an isolation insulating film.
However, if the polysilazane silicon oxide film is used as an isolation insulating film, the polysilazane film is not sufficiently converted into the SiO2 film; for this reason, the following problem arises.
Usually, in a NAND type flash memory, the isolation insulating film is formed, and thereafter, it is etched back thereby controlling the height of the isolation insulating film in a isolation trench. However, if the polysilazane film is not sufficiently converted into the SiO2 film, variations occur in the etching depth. For example, oxygen is not sufficiently supplied in a portion having a narrow trench width; for this reason, the polysilazane film is not sufficiently converted into the SiO2 film. As a result, the etching rate becomes high in the portion having a narrow trench width. Thus, the etching depth increases in the portion having a narrow trench width as compared with a portion having a wide trench width.
As described above, when the insulating film is formed in the trench, variations have conventionally occurred in the etching rate. For this reason, it is difficult to accurately control the etching depth.